Hematite thermodynamic constants

Correction of the thermodynamic constants of hematite raises the need for agreement with the new constants of magnetite. A combination of the linear equation AG=/(T) of the reaction of formation of Fc304 with the experimental measurements (Charette and Flengas, 1968) used earlier (Mel nik,... 

If the initial oxygen/iron ratio is below the stoichiometric value, then some elemental iron will remain, in association with hematite and gaseous oxygen. The /02 for this assemblage would be 10 - bars (as calculated from the equilibrium constant for reaction (19.69) at 25°C). This is an interesting situation, because hematite and iron are thermodynamically metastable under these conditions with respect to magnetite. 

Samples from the site contained considerable amounts of freshly precipitated iron hydroxides. Their transformation into thermodynamically more stable minerals such as goethite or hematite has a very slow kinetics, thus ferrihydrite was chosen as the major adsorbing surface. The Diffuse Double Layer model (Dzombak and Morel, 1990) was selected to describe surface complexation. The respective intrinsic surface parameters and the reaction constants for the ions competing with uranium(VI) for sorption sites were taken from a database mainly based on Dzombak and Morel, 1990, with the urani-um(Vl) sorption parameters as determined by Dicke and Smith, 1996. The results, based on runs with 1000 varied parameter sets, are summarized in Table 5.2. 

Table 111. The Thermodynamics Equilibrium Constant K and 0(CO) of Hematite Reduction...

Thermodynamic Parameters Thermodynamic data for maghemite (y-Fe20g(s)) have been determined in Lemire et al. (2013) and these values are retained in this review. From the data provided by Taylor and Owen (1997), Lemire et al. estimated a difference in the solubility constants of maghemite and hematite (log A gj o°(rii ghemite) - log iiCgj o (hematite) = 1.25 0.46). From the accepted solubility constant determined for hematite in the present review, the calculated solubility constant for maghemite is... 

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